This invention pertains to passive-type range determining systems for measuring the distance (i.e., range) to a source of electromagnetic radiation.
There is often a need, especially in navigational and surveillance operations, to have an effective means for measuring distance to a particular source of electromagnetic radiation, such as radio frequency signals emitted by a radar transmitting antenna. In some situations, the emitter itself may be of a scanning type in which the electromagnetic energy sweeps across the station at which the range measurement is performed. In such case, equipment may be used of the type disclosed in U.S. Pat. No. 4,316,193 issued Feb. 16, 1982, to Philip Jones and myself, and assigned to The Boeing Company of Seattle, Wash. Disclosed in that patent is a system for finding the range to the emitter by measuring a very small time difference signal called t.sub.21, representing the difference in time between the arrival of the scanning emitter at the first, and then at the second of a pair of spaced apart receiving antenna elements, and then producing a signal for range as a function of the time difference t.sub.21.
In other measuring situations, the emitter may be nonscanning such that it is received at the measuring station as a more or less nonmoving stream of radiation. For that case, a related type of ranging equipment may be used as disclosed in U.S. Pat. No. 4,339,755 issued on July 13, 1982, to James M. Wright, and assigned to The Boeing Company. As described in the last mentioned patent, the nonscanning emitter is monitored by a pair of synchronously scanning antennas located at spaced points on the monitoring station with the receiving axes of the scanning antenna elements maintained parallel. At time difference t.sub.21 in the received signals developed by the parallel scanning antennas is processed and used to derive the range measurement. The equipment and technique for this purpose is sometimes called passive ranging on nonscanning emitters or PRONSE.
While the foregoing systems have proven merit, it is desirable to enhance the accuracy of the range measurement and the present invention concerns an improvement to PRONSE systems. One constraint on the accuracy of PRONSE equipment is an unavoidable misalignment of the receiving axes of the antenna beams due, for example, to bending or warping of the platform on which the antenna elements are mounted. Since it is often necessary to measure range to an emitter that is many orders of magnitude greater than the separation of the antenna elements along the base line, any misalignment in the phase relationship of the ideally parallel antenna beams will produce a significant error. Inaccuracy in the range measurement due to this cause is sometimes called a "toe-out" error signifying that one of the antenna beams exhibits toe out relative to the other beam. The platform on which the antennas are mounted is typically on the order of 10 meters or more wide in order to maximize the separation of the antenna elements. With a platform of this magnitude it will unavoidably exhibit mechanical bending or flexure which results in a toe-out error.
Since the measured range of ##EQU2## is based on the ability of the PRONSE system to extract an angle .theta. subtended at the source by the two antennas spaced apart by distance d and scanning at .omega..sub.s, it will be observed that .theta. becomes very small when ranging on emitters that are located hundreds of kilometers from the measurement platform. For example, for d=10 meters and R=100 kilometers and .theta.=0 (i.e., broadside to the antenna array), then the subtended angle .theta. is equal to 10 meters divided by 100 kilometers or 100 microradians. For such very small angles of .theta., the requirement of beam parallelism of the antennas is very demanding. In this example, for instance, a 10 microradian uncompensated departure (toe out) from parallelism of the antenna axes, would result in a 10% error in measured .theta. and, thus, a 10% error in the output signal representing range.